Polyester film, laminate, and package

ABSTRACT

The present invention provides a polyester film suitable for use as a sealant, which not only has high heat sealing strength, but also is less liable to adsorb various organic compounds, and has a less shrinkage in heating and high tensile strength. Further, the present invention provides a laminate comprising at least one layer of the polyester film, and a packaging bag including the same. The polyester film which is formed with a polyester resin whose main constituent is ethylene terephthalate. The film has at least one heat sealing layer. The film has a predetermined heat sealing strength, a predetermined difference in reversing heat capacity measured with a temperature modulated DSC, and a predetermined heat shrinkage. Further, the laminate comprises at least one layer of the polyester film, and the packaging bag includes the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is the U.S. national phase of InternationalPatent Application No. PCT/JP2017/009864, filed Mar. 13, 2017, whichclaims the benefit of Japanese Patent Application No. 2016-055076, filedon Mar. 18, 2016, which are incorporated by reference in theirentireties herein.

TECHNICAL FIELD

The present invention relates to a polyester film which is excellent inheat sealing strength, and a laminate and a packaging bag including thesame.

BACKGROUND ART

Conventionally, sealant films are used as packaging materials for manyof articles on the market represented by foods, pharmaceuticals andindustrial products. The innermost layer of a packaging materialconstituting a packaging bag, a lid member or the like, is provided witha sealant layer made of a polyolefin resin such as polyethylene andpolypropylene, an ionomer, or a copolymer resin such as EMMA whichexhibits high sealing strength. These resins are known to be capable ofachieving high adhesion strength through heat sealing. However, anundrawn sealant film made of a polyolefin resin as described in PatentDocument 1 easily adsorbs a component made of organic compounds such asfat or oil and flavor material, and accordingly, a packaging materialusing a sealant film as the innermost layer, namely, a layer to bebrought into contact with a content, has a disadvantage that the aromaor taste of the content is easily changed. In a case where a sealantlayer made of a polyolefin resin is used as the innermost layer of apackaging bag for a chemical product, a pharmaceutical, a food, or thelike, it is necessary to take such a measure to include a larger amountof active ingredient of a content beforehand, and thus, a sealant layermade of a polyolefin resin is not suitable for the use, in many cases.

On the other hand, a sealant film made of an acrylonitrile resin asdescribed in Patent Document 2 has a feature of being less liable toadsorb an organic compound contained in a chemical product, apharmaceutical, a food or the like. However, an acrylonitrile filmsometimes fails to obtain good sealing strength. In view of suchproblems, Patent Document 3 discloses a polyester film for sealant usewith non-adsorptivity against organic compounds. However, there has beena problem, for example, that when the film of Example 1 of PatentDocument 3 is used as a sealant and left in a high-temperatureenvironment such as a vehicle in mid-summer (about 80° C.), the sealantshrinks and an original shape cannot be maintained. To suppress theshrinking property of the film, there is also an attempt to use anundrawn polyester film as a sealant film. However, an undrawn polyesterfilm has had a problem that, in a case of being subjected to a processsuch as heat sealing or printing, the film cannot withstand tensileforce from a machine, for low tensile strength thereof, and is deformedor broken.

PATENT DOCUMENTS

-   Patent Document 1: JP-A-2002-256116-   Patent Document 2: JP-A-H7-132946-   Patent Document 3: WO02014/175313

NON-PATENT DOCUMENTS

-   Non-Patent Document 1: R. Androsch, B. Wunderlich, Polym., 46,    12556-12566 (2005)-   Non-Patent Document 2: C. Lixon, N. Delpouve, A. Saiter, E.    Dargent, Y. Grohens, Eur. Polym. J., 44, 3377-3384 (2008)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to solve the problems of theconventional technique. That is, a gist of the present invention is toprovide a polyester film suitable for use as a sealant, which not onlyhas high heat sealing strength, but also is less liable to adsorbvarious organic compounds, and has a less shrinkage in heating and hightensile strength. The present invention also intends to provide alaminate comprising at least one layer of the polyester film suitablefor use as a sealant, and a packaging bag including the same.

Means for Solving the Problems

The present invention has the following constitution.

1. A polyester film which is formed with a polyester resin whose mainconstituent is ethylene terephthalate, and satisfies the followingrequirements (1) to (5):

(1) the film has at least one heat sealing layer, and wherein the heatsealing layer is present on at least one of the films surfaces;

(2) the film has a peeling strength of 4 N/15 mm or more and 25 N/15 mmor less when the heat sealing layers of the polyester film areheat-sealed to each other at 160° C. and 0.2 MPa for 2 seconds;

(3) the heat sealing layer of the polyester film has a difference inreversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less,and wherein the difference in reversing heat capacity is measuredbetween at a lower and a higher temperature than a glass transitiontemperature with a temperature modulated DSC;

(4) the film has a heat shrinkage of 0% or more and 15% or less both ina longitudinal direction and in a width direction when treated in hotwater at 80° C. for 10 seconds; and

(5) the film has a tensile breaking strength of 80 MPa or more and 300MPa or less either in a lengthwise direction or in a transversedirection.

2. The polyester film according to above 1, wherein a polyestercomponent constituting the polyester film comprises at least oneselected from the group consisting of neopentyl glycol,1,4-cyclohexanedimethanol, isophthalic acid, and diethylene glycol.

3. The polyester film according to above 1 or 2, wherein a polyestercomponent constituting the polyester film comprises 1,4-butanediol.

4. The polyester film according to any one of above 1 to 3, having afilm thickness of 5 to 200 μm.

5. A packaging bag including at least one of the polyester filmaccording to any one of above 1 to 4.

6. A laminate comprising at least one layer of the polyester filmaccording to any one of above 1 to 4.

7. A packaging bag including at least one of the laminate according toabove 6.

Effects of the Invention

The polyester film of the present invention not only exhibits high heatsealing strength, but also is less liable to adsorb various organiccompounds, and therefore, is capable of hygienically packaging anarticle which contains oil or a flavor material, such as a chemicalproduct, a pharmaceutical, and a food. In addition, the film has a smallshrinkage when heated, and therefore, has a small shrinkage even in ahigh temperature environment. In addition, even when the polyester filmis processed, problem such as breakage is less liable to occur. Thepresent invention is further capable of providing a laminate comprisingat least one layer of the polyester film, and a packaging bag includingthe same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows reversing heat capacity curves of the heat sealing layer(film 1) of Example 1 and the heat sealing layer (film 3) of ComparativeExample 1, as measured by a temperature modulated DSC.

MODE FOR CARRYING OUT THE INVENTION

A polyester film of the present invention is formed with a polyesterresin whose main constituent is ethylene terephthalate, and satisfiesthe following requirements (1) to (5):

(1) the film has at least one heat sealing layer, and wherein the heatsealing layer is present on at least one of the films surfaces;

(2) the film has a peeling strength of 4 N/15 mm or more and 25 N/15 mmor less when the heat sealing layers of the polyester film areheat-sealed to each other at 160° C. and 0.2 MPa for 2 seconds;

(3) the heat sealing layer of the polyester film has a difference inreversing heat capacity of 0.18 J/g·K or more and 0.35 J/g·K or less,and wherein the difference in reversing heat capacity is measuredbetween at a lower and a higher temperature than a glass transitiontemperature with a temperature modulated DSC;

(4) the film has a heat shrinkage of 0% or more and 15% or less both ina longitudinal direction and in a width direction when treated in hotwater at 80° C. for 10 seconds; and

(5) the film has a tensile breaking strength of 80 MPa or more and 300MPa or less either in a lengthwise direction or in a transversedirection.

The polyester film of the present invention which satisfies the aboverequirements is a polyester film excellent in a heat sealing property,and suitable for use as a sealant. Since the film is less liable toadsorb various organic compounds, it is also possible to provide asealant material suitable to a packaging bag. Further, since the filmhas a small shrinkage when heated, it is possible to maintain a shape ofthe film, even in a high temperature environment. In addition, thepolyester film of the present invention has a good processability, forits high tensile strength.

In particular, since each of the heat sealing property and the lowshrinkage; and each of the heat sealing property and the high tensilestrength, are both properties antimonic to each other, there hasconventionally been no polyester film which can satisfy all of theseproperties. Hereinbelow, the polyester film of the present invention isdescribed.

1. Types of the Polyester Raw Materials Constituting the Heat SealingLayer

The polyester used in the heat sealing layer of the present invention isa polyester whose main constituent component is ethylene terephthalateunit.

In addition, it is preferred that the polyester used in the presentinvention contains one or more monomer components which can be amorphouscomponents (hereinbelow simply referred to as amorphous component). Thisis because presence of an amorphous component makes a difference inreversing heat capacity, which will be described later, less liable tolower, even during a film forming process such as drawing or heatsetting, and heat sealing strength is improved. Examples of monomer ofcarboxylic acid component which can be an amorphous component mayinclude isophthalic acid, 1,4-cyclohexanedicarboxylic acid, and2,6-naphthalenedicarboxylic acid.

Further, examples of monomer of a diol component which can be anamorphous component may include neopentyl glycol,1,4-cyclohexanedimethanol, diethylene glycol, 2,2-diethyl1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol,2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, andhexanediol.

Among these amorphous carboxylic acid components and diol components, itis preferred to include isophthalic acid, neopentyl glycol,1,4-cyclohexanedimethanol, or diethylene glycol. Use of these componentsincreases the difference in reversing heat capacity of the film, tofacilitate improvement of heat sealing strength.

In the present invention, it is also possible to include a componentother than ethylene terephthalate or amorphous components. Examples ofdicarboxylic acid component constituting the polyester may includearomatic dicarboxylic acids such as orthophthalic acid; aliphaticdicarboxylic acids such as adipic acid, azelaic acid, sebacic acid anddecanedicarboxylic acid; and alicyclic dicarboxylic acids. However, apolycarboxylic acid having three or more valences (for example,trimellitic acid, pyromellitic acid, and anhydrides thereof) ispreferably not contained in the polyester.

In addition to those described above, examples of a componentconstituting the polyester may include long chain diols such as1,4-butanediol, aliphatic diols such as hexanediol, and aromatic diolssuch as bisphenol A. Among them, 1,4-butanediol is preferably contained.Further, as a component constituting the polyester, a polyesterelastomer containing ε-caprolactone, tetramethylene glycol, or the like,may be contained. These components have not only an effect of increasingthe difference in reversing heat capacity of the film, but also aneffect of lowering the melting point of the film, and are thereforepreferred as a component of the heat sealing layer. However, it ispreferred that the polyester do not contain a diol having 8 or morecarbon atoms (e.g., octanediol, etc.), or a polyalcohol having 3 or morevalences (e.g., trimethylolpropane, trimethylolethane, glycerin,diglycerin, etc.) which lowers film strength significantly.

The polyester film of the present invention may be added with variousadditives such a wax, an antioxidant, an antistatic agent, a crystalnucleating agent, a viscosity reducing agent, a thermal stabilizer, acoloring pigment, an anti-coloring agent, and an ultraviolet absorber.In addition, fine particles as a lubricant for improving smoothness ofthe film are preferably added to at least a surface layer of the film.As the fine particles, those of an arbitrary substance may be selected.Examples of inorganic fine particles may include those of silica,alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate,etc., and examples of organic fine particles may include those ofacrylic resin particles, melamine resin particles, silicone resinparticles, cross-linked polystyrene particles, etc. An average particlesize of the fine particles may be appropriately selected according toneeds within a range of 0.05 to 3.0 μm as measured by Coulter counter.

As a method of blending the particles in the polyester film of thepresent invention, for example, they can be added in an arbitrary stepin production of the polyester resin, but they are preferably added in astep of esterification, or in a step before start of polycondensationreaction and after completion of ester exchange reaction as slurrydispersed in ethylene glycol etc., followed by carrying outpolycondensation reaction. Further, the method may also include such asa method in which slurry of particles dispersed in ethylene glycol,water, other solvent or the like and raw materials of polyester resinare mixed using a kneading extruder with a vent, or a method in whichdried particles and raw materials of polyester resin are mixed using akneading extruder.

2. Layer Structure of the Polyester Film

The polyester film of the present invention includes at least one layerof the heat sealing layer described above, and the heat sealing layer ispresent on at least one of the films surfaces. The polyester film of thepresent invention may be a single layer of a heat sealing layer alone,or may be a laminate structure of two or more layers. In a case wherethe film has a laminate structure, it is necessary that the heat sealinglayer is present on at least one of the films surfaces. In laminatingthe film, it is possible to use a publicly-known method such as: aco-extrusion by a multi-manifold T die or an inflation method; adhesionby a wet or dry lamination or a hot melt; etc. From a viewpoint ofstrength, the heat sealing layer is preferably drawn at least in onedirection (uniaxial drawing), and a biaxial drawing is more preferred,although any of undrawing, uniaxial drawing, and biaxial drawing may beused. A suitable manufacturing method in the case of biaxial drawingwill be described later.

As a raw material used in a layer other than the heat sealing layer, itis possible to use the same polyester as one used in the heat sealinglayer. For example, it is possible to use polyester which contains theaforementioned constituent components suitable to the heat sealinglayer, providing a polyester layer different from the heat sealing layerin compositions. An amount of an amorphous component in a polyester rawmaterial to be used in a layer other than the heat sealing layer ispreferably 25 mol % or less. In a case where the amount of the amorphouscomponent in a layer other than the heat sealing layer is more than 25mol %, the film has to have low mechanical strength and a low heatresistance. From a viewpoint of strength, a layer other than the heatsealing layer is preferably drawn at least in one direction (uniaxialdrawing), and a biaxial drawing is more preferred, although any ofnon-drawing, uniaxial drawing, and biaxial drawing may be used. Asuitable manufacturing method in the case of biaxial drawing will bedescribed later. It is further possible, in the polyester film of thepresent invention, to provide a layer processed by corona treatment,coating treatment, flame treatment or the like, in order to improve anadhesive property of a film surface irrespective of the heat sealinglayer or other layers. Such a layer may be provided arbitrarily within arange not departing from requirements of the present invention.

3. Properties of the Polyester Film

Next, properties of the polyester film of the present invention,required in being used as a sealant, are described.

3.1. Heat Sealing Strength

First, it is preferred that a heat sealing strength of the polyesterfilm of the present invention is 4 N/15 mm or more and 25 N/15 mm orless when the heat sealing layers are heat-sealed to each other at atemperature of 160° C., with a seal bar pressure of 0.2 MPa, for asealing time of 2 seconds.

If the heat sealing strength is less than 4 N/15 mm, a sealed portion iseasily peeled, and the film cannot be used as a packaging bag. The heatsealing strength is preferably 5 N/15 mm or more, more preferably 6 N/15mm or more. Although high heat sealing strength is preferred, acurrently available upper limit is about 25 N/15 mm.

3.2. Difference in Reversing Heat Capacity

It is preferred that the heat sealing layer of the polyester film has adifference in reversing heat capacity of 0.18 J/g·K or more and 0.35J/g·K or less, and wherein the difference in reversing heat capacity ismeasured between at a lower and a higher temperature than a glasstransition temperature (Tg) with a temperature modulated DSC.

Hereinbelow, a relationship between a concept of mobile amorphous phaseand heat sealing strength is described.

Heat sealing is a technique which is worked out by softening orliquefying a heat sealing layer by heating, namely, by changing anarrangement of a polymer which is a constituent component. It is acommon recognition that those cause a softening or liquefying easilywith heat are amorphous components which has a weak binding of amolecular chain. Conventionally, it has been considered that ahigher-order structure of a polymer constituting a film is divided intoa crystalline phase and an amorphous phase, and that an amorphous phaseis increased and heat sealing strength is enhanced simply by increasingan amount of an amorphous component. However, as a result ofinvestigations by the present inventors, it has been found that, in auniaxial drawing film or a biaxial drawing film, even if an amount of anamorphous component is simply increased, no increase in heat sealingstrength corresponding to an increment amount is observed, and that thetendency is particularly remarkable in a biaxial drawing film. Fromthese facts, it was conceived that a mobile amorphous content, whichwill be described below, was contributing to heat sealing strength.

It is known that an amorphous phase is further divide into rigidamorphous phase and mobile amorphous phase according to softness thereof(for example, Non-Patent Document 1). The mobile amorphous phase, whichis the softest component among these three phases, is changed from asolid phase to a liquid phase with Tg as the boundary, in a process oftemperature rise, to increase a heat capacity. On the other hand, in arigid amorphous phase or a crystal phase, change from a solid phase doesnot occur until a melting point is reached, and accordingly, adifference in heat capacity between at a lower and a higher temperaturethan Tg corresponds to a mobile amorphous content. It is consideredthat, when a mobile amorphous content is large, a molecular chain becomeeasy to move, namely, become easy to soften by heat, increasing anintrusion or fusing of the heat sealing layers to each other, resultinghigher heat sealing strength. The inventors of the present inventionhave found that it is possible to secure desirable heat sealing strengthby controlling a mobile amorphous content of a heat sealing layer to bewithin a prescribed range. If the difference in reversing heat capacityΔCp is less than 0.18 J/g·K, a mobile amorphous content necessary to aheat sealing layer cannot be met, and heat sealing strength falls below4 N/15 mm. On the other hand, the higher the difference in reversingheat capacity ΔCp of a heat sealing layer is, the more the heat sealingstrength of the heat sealing layer is increased, which is preferred.

However, when the ΔCp of a heat sealing layer becomes too high, anappropriate heat sealing become difficult, since a heat resistance islowered, and a blocking occurs at the time of heat sealing, in aperiphery of a portion to be sealed (a phenomenon that a region broaderthan intended is sealed due to a heat conduction from a heating member).An upper limit of preferred difference in reversing heat capacity ΔCp is0.4 J/g·K.

3.3. Shrinkage

A hot-water heat shrinkage in the polyester film of the presentinvention is 0% or more and 15% or less both in the width direction andthe longitudinal direction, when treated in hot water of 80° C. for 10seconds.

When the shrinkage exceeds 15%, a shrinkage becomes large when a film isheat-sealed, deteriorating flatness after the sealing. An upper limit ofthe hot-water heat shrinkage is preferably 14% or less, more preferably13% or less. On the other hand, a hot-water heat shrinkage below zeromeans that a film is elongated, which is not preferred because itbecomes hard to maintain an original shape of a film, similarly in thecase where a shrinkage is high.

3.4. Film Thickness

A thickness of the polyester film of the present invention is preferably3 μm or more and 200 μm or less, although not particularly limited. Afilm thickness thinner than 3 μm is not preferred for causing a lack ofheat sealing strength or making a process such as a printing difficult.A film thickness of thicker than 200 μm may also be possible, but is notmuch preferred because it increases weight of the film to be used, toincrease a chemical cost. The film thickness is more preferably 5 μm ormore and 160 μm or less, and even more preferably 7 μm or more and 120μm or less.

3.5. Thickness Irregularity in the Longitudinal Direction

A thickness irregularity of the polyester film of the present inventionis preferably 18% or less when a measured length is 10 m in thelongitudinal direction. A value exceeding 18% of the thicknessirregularity in the longitudinal direction is not preferred, because ittends to cause a printing error when the film is subjected to aprinting. Incidentally, the thickness irregularity in the longitudinaldirection is more preferably 16% or less, and particularly preferably14% or less. Although the smaller the thickness irregularity in thelongitudinal direction is, the more it is preferred, the lower limitthereof is considered to be about 1% at the minimum, in view ofperformance of a film forming apparatus.

3.6. Thickness Irregularity in the Width Direction

In the width direction, thickness irregularity is preferably 18% orless, when a measured length is 1 m. A value exceeding 18% of thethickness irregularity in the width direction is not preferred, becauseit tends to cause a printing error when the film is subjected to aprinting. Incidentally, the thickness irregularity in the widthdirection is more preferably 16% or less, and particularly preferably14% or less. Although the nearer to 0% the thickness irregularity in thewidth direction is, the more it is preferred, 1% is considered to beadequate for the lower limit, in view of performance of a film formingapparatus and easy producibility

3.7. Tensile Breaking Strength

A tensile breaking strength of the polyester film of the presentinvention is preferably 80 MPa or more and 300 MPa or less, at least ineither the longitudinal direction or the width direction of the film. Atensile breaking strength below 80 MPa is not preferred, because ittends to cause a deformation or a breakage when a process such as a heatsealing or a printing is conducted. The tensile breaking strength ismore preferably 90 MPa or more, and even more preferably 100 MPa ormore. Although the higher the tensile breaking strength is, the toughera film becomes, which is preferred, in the polyester film of the presentinvention it is difficult to exceed 300 MPa, and accordingly, un upperlimit is 300 MPa.

4. Manufacturing Method of the Polyester Film

4.1. Film Forming by Co-Extrusion

4.1.1. Melt Extrusion

The polyester film of the present invention is produced by usingpolyester as described in the above 1. “Types of the polyester rawmaterials constituting the heat sealing layer” as a raw material. In thepresent invention, it is possible to use, as a sealant layer, an undrawnfilm obtained by melt-extruding a polyester raw material. However inthis case, in order to enhance strength of the film, it is preferredthat the film is laminated with a polyester film drawn in at least onedirection by the method listed in the above 2. “Layer structure of thepolyester film”. In the present invention, it is also possible to obtainthe polyester film by subjecting an undrawn film obtained bymelt-extruding a polyester raw material to a uniaxial drawing or abiaxial drawing according to a prescribed method described below. In acase of providing a layer in addition to a sealant layer at the time ofmelt extrusion, undrawn laminated films are obtained by conducting meltextrusion by separate extruder for individual layer. In this connection,it is possible to obtain polyester so as to contain an appropriateamount of monomers which can be amorphous components as described above,by selecting kind and amount of a dicarboxylic acid component and a diolcomponent, which are then polycondensed. It is also possible that two ormore kinds of chip-shaped polyesters are mixed to be used as rawmaterials of the film. When a raw material is melt-extruded, it ispreferable to dry the polyester raw material using a dryer such as ahopper dryer and a paddle dryer, or a vacuum dryer. After the polyesterraw material is dried in such a manner, utilizing an extruder, it ismelted at a temperature of 200 to 300° C., and extruded into an undrawnfilm. In such an extrusion, an arbitrary conventional method such as aT-die method and a tubular method can be adopted.

Then, the sheet-like molten resin after extrusion is quenched to be ableto obtain an undrawn film. As a method for quenching the molten resin, amethod in which a molten resin is cast on a rotary drum from a die andsolidified by quenching to obtain a substantially unoriented resin sheetcan be suitably adopted. The film is preferably drawn in at least onedirection of lengthwise (longitudinal) direction and transverse (width)direction, that is, uniaxial drawing or biaxial drawing is preferred.Below described is a sequential-biaxial drawing method performed aslengthwise drawing-transverse drawing that first performs a lengthwisedrawing and next performs a transverse drawing. The method may also beperformed as transverse drawing—lengthwise drawing that reverses theorder, since it merely changes the direction of main orientation. It isalso possible to use a simultaneous biaxial drawing method.

4.1.2. Lengthwise Drawing

The lengthwise drawing may preferably be performed by introducing anundrawn film to a lengthwise drawing machine in which a plural number ofrolls are continuously disposed. In a lengthwise drawing, it ispreferred that preheating be performed by a preheating roll, until afilm temperature reaches 65° C. to 90° C. The film temperature lowerthan 65° C. makes a drawing in the lengthwise direction difficult, andaccordingly, breakage tends to occur, which is not preferred. Thetemperature higher than 90° C. makes the film liable to stick to a roll,making the film liable to wind around a roll or making a roll liable tobe contaminated from a successive production, which is not preferred.

A lengthwise drawing is conducted when the film temperature has reached65° C. to 96° C. A lengthwise drawing ratio is preferably 1 time or moreand 5 times or less. Since the 1 time means that lengthwise drawing hasnot been performed, the lengthwise drawing ratio should be 1 time inorder to obtain a transverse uniaxial drawn film, and the lengthwisedrawing should be 1.1 times or more in order to obtain a biaxial drawnfilm. Although an upper limit of the lengthwise drawing ratio may be anytimes, a too high lengthwise drawing ratio makes a transverse drawingdifficult, making a breakage easily occur. Therefore, the upper limit ispreferably 5 times or less. In addition, after the lengthwise drawing, ashrinkage in the longitudinal direction of the film, caused by thelengthwise drawing, may be reduced by relaxing the film in thelongitudinal direction (relaxation in the longitudinal direction). It isalso possible, by the relaxation in the longitudinal direction, toreduce a bowing phenomenon (distortion) which occurs in a tenter. Sincethe polyester film of the present invention contains an amorphousmaterial, a shrinking property in the longitudinal direction caused bythe lengthwise drawing is considered dominant to bowing distortion. Thatis because, in the subsequent steps of transverse drawing or the finalheat treatment, heating is performed in such a state that both ends inthe film width direction are held, and therefore, only the centerportion of the film shrinks in the longitudinal direction. A relaxationratio in the longitudinal direction is preferably 0% or more and 70% orless (relaxation ratio 0% means that relaxation is not conducted). Sincean upper limit of the relaxation ratio in the longitudinal direction isdetermined depending on materials to be used or conditions in thelengthwise drawing, it is impossible to carry out relaxation beyond theupper limit. In the polyester film of the present invention, an upperlimit of the relaxation ratio in the longitudinal direction is 70%.Relaxation in the longitudinal direction may be carried out by heatingthe film after the lengthwise drawing at a temperature of 65° C. to 100°C., and controlling the difference in rotation speed between the rolls.As a means of the heating, it is possible to use any of a roll, a nearinfrared ray, a far infrared ray, a hot air heater, and the like. Therelaxation in the longitudinal direction is not necessarily carried outafter the lengthwise drawing, but may be carried out at an arbitrarytiming. For example, it is possible to carry out the relaxation duringthe transverse drawing (including a preheating zone) or during the finalheat treatment, by narrowing a clip interval in the longitudinaldirection (in this case, both ends in the film width direction are alsorelaxed in the longitudinal direction, and accordingly, the bowingdistortion is reduced). After the relaxation in the longitudinaldirection (in a case where the relaxation is not conducted, thelengthwise drawing), the film preferably be cooled once, and preferablybe cooled on a cooling roll having a surface temperature of 20 to 40° C.

4.1.3. Transverse Drawing

It is preferred that, after the lengthwise drawing, a transverse drawingis conducted with holding both ends in the width direction of the filmby clips in a tenter, at 65° C. to 110° C., at a drawing ratio of 3.5 to5 times. It is preferred that preheating has been conducted before thetransverse drawing, and that the preheating has been conducted until asurface temperature of the film reaches 75° C. to 120° C.

It is preferred that, after the transverse drawing, the film is passedthrough an intermediate zone in which no aggressive heating operation iscarried out. The subsequent final heat treatment zone has a highertemperature against the transverse drawing zone in the tenter.Therefore, if an intermediate zone is not provided, heat of the finalheat treatment zone (hot air itself or radiant heat) should flow intothe transverse drawing step. In such a case, a temperature of thetransverse drawing zone is not stabilized, to cause not only adeterioration in the film thickness accuracy, but also a variation in aphysical property such as heat sealing strength and a shrinkage. Thus,it is preferred that the film after the transverse drawing is passedthrough the intermediate zone to allow a prescribed time to elapse, andthereafter, the final heating treatment is conducted. In thisintermediate zone, it is important to block an associated flow thataccompanies a running of a film, or hot air from the transverse drawingzone or the final heat treatment zone, such that, when a strip-shapedpaper piece is suspended in the intermediate zone in a state where afilm is not passed through, the paper piece hangs down almost completelyin the vertical direction. For a passage time of the intermediate zone,about 1 second to 5 seconds is sufficient. If the passage time isshorter than 1 second, the length of the intermediate zone becomesinsufficient, resulting in an insufficient heat blocking effect. On theother hand, if the intermediate zone is too long, although the longerthe more preferred, equipment should become large. Therefore, about 5seconds is sufficient for the length.

4.1.4. Final Heat Treatment

After passed through the intermediate zone, the film is preferablysubjected to a heat treatment at a temperature equal to or higher thanthe transverse drawing temperature and equal to or less than 180° C. inthe final heat treatment zone. When the heat treatment temperature isnot equal to or higher than the transverse drawing temperature, effectof the heat treatment is not exerted. In such a case, a shrinkage in hotwater of 80° C. becomes higher than 15%, which is not preferred.Although the higher the heat treatment temperature becomes, the lowerthe shrinkage becomes, if the temperature becomes higher than 180° C., ahaze of the film will exceed 15%, and the film will not be able tomaintain transparency, which is not preferred. In the final heattreatment, it is possible to reduce a shrinkage in the width direction,by shortening a distance between the clips of the tenter at an arbitraryratio (relaxation in the width direction). Thus, in the final heattreatment, it is preferred that relaxation in a range of 0% or more and10% or less in the width direction be conducted (relaxation ratio 0%means that relaxation is not conducted). Although the higher arelaxation ratio in the width direction is, the more a shrinkage in thewidth direction lowers, an upper limit of the relaxation ratio (ashrinkage in the width direction of a film immediately after atransverse drawing) is determined depending on raw materials used,drawing conditions in the width direction, or heat treatmenttemperature, and relaxation cannot be carried out beyond the upperlimit. In the polyester film of the present invention, an upper limit ofthe relaxation ratio in the width direction is 10%. A time of passingthrough the final heat treatment zone is preferably 2 seconds or moreand 20 seconds or less. If the passing time is less than 2 seconds, thefilm passes through the heat treatment zone without having a surfacetemperature reached a set temperature, which makes the heat treatmentmeaningless. The passing time is preferably 2 seconds or more, and morepreferably 5 seconds or more, since the longer the passing time, themore the effect of the heat treatment is achieved. However, a longerpassing time will result in an immense size of equipment, and 20 secondsor less is sufficient for a practical use. After that, the film iswound, with both ends cut and removed, to thus obtain a polyester filmroll.

4.2. Adhesion Between Films

When the polyester film of the present invention is produced, it is alsopossible to adhere the polyester film mentioned in the above 4.1 “Filmforming by co-extrusion” to another polyester film. When an undrawn filmis used as the heat sealing layer, it is preferably adhered to apolyester film drawn in at least one direction. As a method of adhesion,the method listed in the above 2. “Layer structure of polyester film”may be used. In a case of dry lamination, it is possible to use acommercially available adhesive for dry lamination. Representativeexamples are Dick Dry (registered trademark) LX-703VL manufactured byDIC Corporation, KR-90 manufactured by DIC Corporation, TAKENATE(registered trademark) A-4 manufactured by Mitsui Chemicals, Inc.,TAKELAC (registered trademark) A-905 manufactured by Mitsui Chemicals,Inc., etc.

EXAMPLES

Next, the present invention will be described more concretely by way ofExamples and Comparative Examples. However, the present invention is byno means limited by modes of the Examples, and may appropriately bemodified within a range not deviated from the gist of the presentinvention.

Evaluation methods of the film are as follows. Incidentally, in a casewhere a longitudinal direction and a width direction cannot immediatelybe identified for such a reason that an area of a film is small, thelongitudinal direction and the width direction may provisionally bedetermined, since no problem is particularly caused even if theprovisionally determined longitudinal direction and width direction aredeviated by 90 degrees from correct directions.

<Evaluation Methods of Heat Sealing Layer>

[Mobile Amorphous Content]

Samples of the heat sealing layers of 10.0±0.2 mg were weighed and putinto a hermetic aluminum pan, and reversing heat capacity curves thereofwere obtained by using a temperature modulated differential scanningcalorimeter (DSC) “Q100” (manufactured by TA Instruments) in MDSC(registered trademark) heat-only mode, at an average temperature raisingspeed of 2.0° C./min., and with a modulation period of 60 seconds.

When a sealant layer is obtained from a laminated film, in a case ofco-extruded film, a surface layer in a side of heat sealing layer wasshaved with a feather blade. A cross section of the shaved film samplewas observed with an electron scanning microscope (SEM) to check anylayer other than heat sealing layer was not shaved. In a case of thelaminate film, a notch was made in the film, from which the film wastorn by hand, and interlayer exfoliation in the torn portion (cut) waspeeled off with tweezers. For the peeled off heat sealing layer, aportion 1 cm or more away from the cut was sampled.

In the heat capacity curve obtained by the measurement, an inflectionpoint was determined by using an attached analysis software (TAAnalysis, manufactured by TA Instruments), and as a difference in heatcapacity between at a lower and a higher temperature than the inflectionpoint (a glass transition point), a difference in reversing heatcapacity x ΔCp was determined according to the following equation 1.Difference in reversing heat capacity ΔCp=(Heat capacity Cp1 in hightemperature side)−(Heat capacity Cp2 in low temperature side)  Equation1

Here, in the heat capacity curve, an extension line of the base line ofthe heat capacity curve in the high temperature side relative to Tg wasdrawn, and an intersection point thereof with a tangent line of theinflection point (Tg) was determined. A value of Y axis corresponding tothe intersection point (a reversing heat capacity) was read, andregarded as a heat capacity Cp1 in the high temperature side. Anextension line of the base line of the heat capacity curve in the lowtemperature side relative to Tg was also drawn, and an intersectionpoint thereof with a tangent line of the inflection point (Tg) wasdetermined. A value of Y axis corresponding to the intersection point (areversing heat capacity) was read, and regarded as a heat capacity Cp2in the low temperature side.

<Evaluation Methods of Film>

[Heat Sealing Strength]

Heat sealing strength was measured in conformity with JIS Z1707.Specific procedure is briefly shown. Heat sealing layers of sampleswhich have not been subjected to a coating treatment or a coronatreatment are adhered to each other with a heat sealer. Sealingconditions were an upper bar temperature set to 160° C., a lower bartemperature set to 100° C., a pressure was set to 0.2 MPa, and a timewas set to 2 seconds. Adhesion samples were cut out so as to have a sealwidth of 15 mm. Peeling strength was measured by using a universaltensile strength tester “DSS-100” (manufactured by Shimadzu Corporation)at a tensile speed of 200 mm/minute. The peeling strength is shown instrength per 15 mm (N/15 min).

[Hot-Water Heat Shrinkage]

A film was cut into a square of 10 cm×10 cm, shrunk by being immersed inhot water of 80±0.5° C. for 10 seconds under a no load condition, thenimmersed in water of 25° C.±0.5° C. for 10 seconds, and taken out fromthe water. Thereafter, dimensions of the film in the lengthwisedirections and transverse directions were measured, and a shrinkage ineach directions was determined according to the following equation 2.Incidentally, the measurement was conducted twice, and average thereofwas determined.Shrinkage={(Length before shrinkage−Length after shrinkage)/Lengthbefore shrinkage}×100(%)  Equation 2[Appearance After Left in High Temperature Environment]

A film was cut into a square of 10 cm×10 cm, and then laminated withanother biaxial drawing polyester film E5100-12 μm (manufactured byToyobo Co., Ltd.) similarly cut into a square of 10 cm×10 cm, by usingan adhesive for dry lamination (TAKELAC (registered trademark) A-950manufactured by Mitsui Chemicals, Inc.). This laminate was put in athermo-hygrostat (IG400 manufactured by Yamato Scientific Co., Ltd.) inwhich temperature and humidity were set to 80° C./65% RH, and left for24 hours. After 24 hours, the laminate was taken out, and the shrinkagedetermined from the above equation 2 was calculated. In a case that theshrinkage varies depending on a direction of the film, a shrinkage of adirection which had a larger shrinkage was taken. This shrinkage wasevaluated as follows as an appearance after left in a high-temperatureenvironment.

Judgment good shrinkage ratio relative to the original shape was lessthan 2%.

Judgment fair shrinkage ratio relative to the original shape was 2% ormore and 5% or less.

Judgment poor shrinkage ratio relative to the original shape was morethan 5%.

[Thickness Irregularity in Longitudinal Direction]

A film was sampled in a shape of roll of 11 m in the longitudinaldirection×40 mm in the width direction, and the thickness was measuredcontinuously along the longitudinal direction of the film (measuredlength was 10 m), by using a continuous contact-type thickness metermanufactured by Micron Measuring Instrument Co., Ltd, at a measuringspeed of 5 m/min. The thickness irregularity in the longitudinaldirection of the film was calculated from the following equation 3, inwhich Tmax. is a maximum thickness, Tmin. is a minimum thickness, andTave. is an average thickness at the time of measurement.Thickness irregularity={(Tmax.−Tmin.)/Tave.}×100(%)   Equation 3[Thickness Irregularity in Width Direction]

A film was sampled in a shape of wide belt of 40 mm length×1.2 m width,and thickness was measured continuously along the width direction of thefilm sample (measured length was 1 m), by using a continuouscontact-type thickness meter manufactured by Micron Measuring InstrumentCo., Ltd, at a measuring speed of 5 m/min. Thickness irregularity in thelongitudinal direction of the film was calculated from the aboveequation 3, wherein Tmax. is a maximum thickness, Tmin. is a minimumthickness, and Tave. is an average thickness at the time of measurement.

[Tensile Breaking Strength]

A film sample in a rectangular shape of 140 mm in a direction to bemeasured and 20 min in a direction perpendicular to the measurementdirection was prepared in conformity with JIS K7113. A tensile test wasconducted by using a universal tensile tester “DSS-100” (manufactured byShimadzu Corporation), with holding each 20 mm of both ends of a testpiece with chucks (100 mm distance between chucks), under conditions ofan ambient temperature of 23° C. and a tensile speed of 200 mm/min., anda stress at the time of tensile failure was taken as tensile breakingstrength (MPa). Measurement directions were longitudinal direction andwidth direction, in which test was conducted individually.

[Aroma Retainability]

Two pieces of films were cut into squares of 10 cm×10 cm, which werethen laminated with each other, and formed into a bag, with only oneside open, and three sides heat-sealed at 160° C. Each 20 g of limonene(manufactured by NACALAI TESQUE, INC.) and menthol (manufactured byNACALAI TESQUE, INC.) were put therein, and then the opening one sidewas also heat sealed to produce a sealed bag. The bag was put in a glasscontainer with a capacity of 1000 ml, and a lid was put on the glasscontainer. After one week, the lid of the glass container was opened sothat persons (total 16 persons with 4 persons in twenties, 4 persons inthirties, 4 persons in forties and 4 persons in fifties; the ratio ofmen to women was 1:1 in each age category) could smell the air in theglass container. The air in the glass container was smelled, andevaluations were made as follows.

Judgment good the number of persons who sensed a smell 0 to 1

Judgment fair the number of persons who sensed a smell 2 to 3

Judgment poor the number of persons who sensed a smell 4 to 16

[Adsorptivity]

A film was cut into a square of 10 cm×10 cm, and weighed. Then, the filmwas soaked into a solution of 500 ml in a container which was preparedby adding ethanol to limonene (manufactured by NACALAI TESQUE, INC.) andmenthol (manufactured by NACALAI TESQUE, INC.) so that concentration ofeach became 30%, and taken out a week later. The film taken out waspressed with a Bemcot to remove the solution, and dried for 1 day in aroom of a temperature of 23° C. and humidity of 60% RH. After dried, thefilm was weighed, and a difference in film weight determined from thefollowing equation 3 was defined as an adsorbed amount.Adsorbed amount=Film weight after immersion−Film weight beforeimmersion  Equation 3

The adsorbed amount was judged as follows.

Judgment good 0 mg or more and 5 mg or less

Judgment fair more than 5 mg and equal to or less than 10 mg

Judgment poor more than 10 mg

<Preparation of Polyester Raw Material>

Synthesis Example 1

A stainless steel autoclave equipped with a stirrer, a thermometer, anda partial reflux condenser was charged with 100 mol % of dimethylterephthalate (DMT) as a dicarboxylic acid component, and 100 mol % ofethylene glycol (EG) as a polyalcohol component, such that, the ethyleneglycol became 2.2 times of the dimethyl terephthalate in molar ratio,and then a transesterification reaction was conducted by using 0.05 mol% (relative to the acid component) of zinc acetate as atransesterification catalyst, with distilling off generated methanolfrom the system. Thereafter, 0.225 mol % (relative to the acidcomponent) of antimony trioxide was added as a polycondensationcatalyst, and a polycondensation reaction was conducted at 280° C.,under a condition of reduced pressure of 26.7 Pa, to obtain polyester(A) with intrinsic viscosity of 0.75 dl/g. This polyester (A) is apolyethylene terephthalate.

Synthesis Example 2

Polyesters (B) to (F) were obtained in the same procedure as theSynthesis example 1, with changing the monomer. The compositions of eachpolyester are shown in Table 1. In Table 1, TPA is terephthalic acid,IPA is isophthalic acid, BD is 1,4-butanediol, NPG is neopentyl glycol,CHDM is 1,4-cyclohexanedimethanol, and DEG is diethylene glycol. In theproduction of polyester (E), SiO2 (Sylysia 266 manufactured by FujiSilysia Ltd.) was added as a lubricant in a ratio of 7,000 ppm relativeto the polyester. Each polyester was appropriately formed into a chip.The intrinsic viscosities of each polyester was B: 0.78 dl/g, C: 0.73dl/g, D: 0.73 dl/g, E: 0.80 dl/g, F: 0.75 dl/g, respectively.

TABLE 1 composition of polyester raw material (mol %) dicarboxylicaddition polyester acid amount of raw component diol component lubricantmaterial TPA IPA EG BD NPG CHDM DEG (ppm) A 100 0 99 0 0 0 1 0 B 80 2099 0 0 0 1 0 C 100 0 68 0 30 0 2 0 D 100 0 67 0 0 30 3 0 E 100 0 0 100 00 0 0 F 100 0 99 0 0 0 1 7000

The production method of each film is described below.

(Production of Polyester Film 1)

Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 5:66:24:5, as raw materials of a heatsealing layer (layer A). The mixed raw materials for layer A were fedinto a biaxial screw extruder, melted at 270° C., discharged from a Tdie, and cooled on a chill roll with a surface temperature set to 30°C., to obtain an undrawn film formed only with layer A. The undrawn filmobtained by cooling and solidification was introduced to a lengthwisedrawing machine in which a plural number of rolls were arrangedcontinuously, preheated on a preheating roll until the film temperaturereached 80° C., and then drawn 4.1 times. The film immediately after thelengthwise drawing was passed through a heating furnace set by a hot airheater to 90° C., and subjected to a 20% relax treatment in thelongitudinal direction, by utilizing the difference in rotation speedbetween the rolls at an entrance and an exit of the heating furnace.Thereafter, the lengthwise drawing film was forcibly cooled by a coolingroll with a surface temperature set to 25° C. The film after the relaxtreatment was introduced to a transverse drawing machine (tenter),subjected to preheating of 5 seconds until the surface temperaturereached 95° C., and then drawn 4.0 times in the width direction (thetransverse direction). The film after the transverse drawing wasdirectly introduced to the intermediate zone, through which the film waspassed through in 1.0 second. In this connection, in the intermediatezone of the tenter, hot air from the final heat treatment zone and hotair from the transverse drawing zone were blocked, such that, when arectangular shaped piece of paper was dropped in the intermediate zonein a state where a film was not passed through, the piece of paperdropped in a substantially completely perpendicular direction. Afterthat, the film passed through the intermediate zone was introduced tothe final heat treatment zone, and subjected to a heat treatment at 125°C. for 5 seconds. At this time, a relaxation treatment of 3% in thewidth direction was conducted, by narrowing a clip interval in the filmwidth direction, simultaneously with the heat treatment. After passedthrough the final heat treatment zone, the film was cooled, and woundinto a roll shape with both ends cut and removed, so as to have a widthof 500 mm, to produce a biaxial drawn film with a thickness of 30 μmcontinuously over a prescribed length. The production conditions areshown in Table 2.

(Production of Polyester Film 2)

Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 5:78:12:5, as raw materials of layer A.Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 5:54:36:5, as raw materials of the otherlayer (layer B). The raw material mixtures of layer A and layer B wereeach fed to separate biaxial screw extruders, and were melted at 270° C.Each molten resin was joined together by a feed block in the middle of aflow path, and discharged from a T die, and cooled on a chill roll witha surface temperature set to 30° C., to obtain an un drawn laminatedfilm. The flow path for the molten resin was configured such that layerA formed both surface layers, and layer B formed the center layer of thelaminated film (a 2 kind-3 layer structure of layer A/layer B/layer A),and an amount of discharge was controlled such that a thickness ratio oflayer A and layer B became 50:50. Then, the film was formed into abiaxial drawn film with a width of 500 mm and a thickness of 30 μm,under the same conditions with polyester film 1 except that the finalheat treatment temperature was set to 140° C. and the relaxation in thewidth direction was set to 15%. The production conditions are shown inTable 2.

(Production of Polyester Film 3)

An undrawn laminated film was obtained by using the same raw materialsin the same mixing ratios with those of polyester film 2, for both layerA and layer B, which were melt-extruded in the same manner as in Example2. Thereafter, the film was formed into a biaxial drawn film with awidth of 500 mm and a thickness of 30 μm, under all the same conditionswith polyester film 1, except that the final heat treatment temperaturewas set to 100° C. and the relaxation ratio in the width direction wasset to 0%. The production conditions are shown in Table 2.

(Production of Polyester Film 4)

Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 10:75:10:5, as raw materials of layer A.Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 55:30:10:5, as raw materials of the otherlayer (layer B). These raw materials were each melt-extruded in the samemanner as polyester film 2, to obtain am undrawn laminated film.Thereafter, the film was formed into a biaxial drawn film with a widthof 500 mm and a thickness of 30 μm, under all the same conditions withExample 1, except that the lengthwise drawing temperature was set to 90°C., the transverse drawing temperature was set to 100° C., and the finalheat treatment temperature was set to 115° C. The production conditionsare shown in Table 2.

(Production of Polyester Film 5)

Polyester A, polyester C, polyester E, and polyester F were mixedtogether at a mass ratio of 32:53:10:5, as raw materials of layer A, andthe mixture was melt-extruded in the same manner as polyester film 1 toobtain an undrawn film formed only with layer A. Thereafter, the filmwas formed into a biaxial drawn film with a width of 500 mm and athickness of 30 μm, under the same conditions with polyester film 4. Theproduction conditions are shown in Table 2.

(Production of Polyester Film 6)

Polyester A, polyester B, polyester D, polyester E, and polyester F weremixed together at a mass ratio of 5:86:2:2:5, as raw materials of layerA, and the mixture was melt-extruded in the same manner as polyesterfilm 1, to obtain an undrawn film formed only with layer A. The film waswound into a roll shape with the both ends cut and removed, so as tohave a width of 500 mm, to obtain an undrawn film having a thickness of20 μm. The production conditions are shown in Table 2.

(Production of Polyester Film 7)

Polyester C and polyester F were mixed together at a mass ratio of 95:5as raw materials of layer A, which were formed into a film under thesame conditions with polyester film 6 in the subsequent steps, to obtainan undrawn film formed only with layer A. The production conditions areshown in Table 2.

(Production of Polyester Film 8)

Polyester A and polyester F were mixed together at a mass ratio of 95:5as raw materials of layer B, and the mixture was melt-extruded by thesame method with polyester film 1. Thereafter, a lengthwise drawing wasconducted by the same method with polyester film 4, and the film wasintroduced to a tenter without subjected to a relaxation treatment inthe longitudinal direction. Thereafter, the film was formed into abiaxial drawn film formed only with layer B, under the same conditionswith polyester film 1 except that the transverse drawing temperature wasset to 110° C., and the final heat treatment temperature was set to 220°C. The production conditions are shown in Table 2.

TABLE 2 film 1 film 2 film 3 film 4 film 5 film 6 film 7 film 8 Rawmaterial polyester A 5 5 5 10 32 5 0 — composition polyester B 0 0 0 0 086 0 — of heat polyester C 66 78 78 75 53 0 95 — sealing layer polyesterD 0 0 0 0 0 2 0 — (A layer) polyester E 24 12 12 10 10 2 0 — (mass %)polyester F 5 5 5 5 5 5 5 — Raw material polyester A — 5 5 55 — — — 95composition of polyester B — 0 0 0 — — — 0 layer (B layer) polyester C —54 54 30 — — — 0 other than polyester D — 0 0 0 — — — 0 heat sealingpolyester E — 36 36 10 — — — 0 layer (mass %) polyester F — 5 5 5 — — —5 Layer structure A A/B/A AB/A A/B/A A A A B Thickness ratio of layer(layer A (%)/ 100/0 50/50 50/50 50/50 100/0 100/0 100/0 0/100 layer B(%)) Lengthwise drawing 80 80 80 90 90 — — 90 drawing temperature (° C.)drawing ratio 4.1 4.1 4.1 4.1 4.1 — — 4.1 Relaxation heating furnace 9090 90 90 90 — — — treatment in the temperature (° C.) longitudinalrelaxation ratio (%) 20 20 20 20 20 — — — direction Transverse drawing95 95 95 100 100 — — 110 drawing temperature (° C.) drawing ratio 4.04.0 4.0 4.0 4.0 — — 4.0 Final heat temperature (° C.) 125 140 100 115115 — — 220 treatment relaxation ratio in 3 15 0 3 3 — — 3 the widthdirection (%) Film thickness (μm) 30 30 30 30 30 20 20 10[Evaluation of Film]

Example 1

In Example 1, an evaluation film was produced by laminating polyesterfilm 1 as the heat sealing layer, and polyester film 8 as the otherlayer. The two films were adhered to each other by using an adhesive fordry lamination (TAKELAC (registered trademark) A-950 manufactured byMitsui Chemicals, Inc.). Evaluation results of the obtained laminatedfilm are shown in Table 3.

Examples 2 to 4

In Examples 2 to 4, the heat sealing layer and the other layer were alsolaminated with each other by the same method with Example 1. Polyesterfilms 3, 6, and 7 were used as heat sealing layers of Examples 2, 3, and4, respectively, and polyester film 8 was used as the other layer ofeach. Evaluation results of the obtained laminated films are shown inTable 3.

Examples 5 to 8

In Examples 5, 6, and 8, polyester films 1, 2, 4, and 5 wererespectively used as a single layer. Evaluation results of each film areshown in Table 3.

Comparative Examples 1 to 3

In Comparative Examples 1, 2, and 3, polyester films 3, 6, and 8 wererespectively used as a single layer. Evaluation results of each film areshown in Table 3.

TABLE 3 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ComparativeComparative Comparative ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8Example 1 Example 2 Example 3 Layer heat sealing film 1 film 3 film 6film 7 film 1 film 2 film 4 film 5 film 3 film 6 film 8 structure layerother layer film 8 film 8 film 8 film 8 — — — — — — — Total thickness(μm) 40 40 30 30 30 30 30 30 30 20 10 Difference in reversing heat 0.260.22 0.29 0.34 0.26 0.24 0.22 0.18 0.22 0.29 0.13 capacity of heatsealing layer ΔC_(p) (J/g · K) Heat sealing strength at 160° 15.1 14.318.1 22.8 14.6 13.1 12.2 5.1 Unmea- 18.6 0 C. (N/15 mm) surableHot-water Lengthwise 1.8 0.8 0.3 0.7 5.5 1.8 4.8 4.8 9.4 0.5 0.1shrinkage (%) direction 80° C. Transverse 4.4 1.2 0.1 0.2 13.5 −1.5 6.86.6 23.1 0.4 0.2 direction Thickness Lengthwise 14.3 13.7 10.5 9.4 13.512.4 3.2 3.0 13.1 8.4 2.5 irregularity (%) direction Transverse 16.814.9 11.3 8.8 15.6 14.6 5.4 5.5 15.3 7.4 4.7 direction TensileLengthwise 167 159 97 102 139 142 220 224 122 63 247 breaking directionstrength (MPa) Transverse 185 188 103 104 169 164 256 258 149 61 245direction Appearance after left in high good good good good fair fairfair fair poor good good temperature environment Flavor menthol goodgood good good good good good good good good good retainability limonenegood good good good good good good good good good good Adsorptivitymenthol good good good good good good good good good good good limonenegood good good good good good good good good good good[Evaluation Results of Films]

According to Table 3, good evaluation results were obtained in any ofthe films of Examples 1 to 4, with a difference in reversing heatcapacity ΔCp of the heat sealing layer in a prescribed range, andexcellent heat sealing strength, shrinkage, thickness irregularity;tensile breaking strength, appearance after left in a high-temperatureenvironment, aroma retainability, and adsorptivity. Incidentally, thechart in FIG. 1 shows no disturbance, and the base line shifts in thevicinity of Tg, which confirms that the DSC measurement was performedproperly. As for the films of Examples 5 to 8, although the appearanceafter left in a high-temperature environment was somewhat inferior tothose of Examples 1 to 4, the heat sealing strength were measurable, andthe films were found to have no problem in use. Any of the films ofExamples 5 to 8 exhibited a difference in reversing heat capacity ΔCp ofthe heat sealing layer in a prescribed range, and was excellent in heatsealing strength, shrinkage, thickness irregularity, tensile breakingstrength, aroma retainability, and adsorptivity. On the other hand, asfor the film of Comparative Example 1, although the ΔCp of the heatsealing layer was satisfied, and thickness irregularity, tensilebreaking strength, aroma retainability, and an adsorptive property wereexcellent, it was impossible to measure heat sealing strength, forhaving poor flatness after the heat sealing, due to a high shrinkage.Moreover, evaluation on appearance after left in high temperatureenvironment was also low. As for the film of Comparative Example 2,although the ΔCp of the heat sealing layer was also satisfied, and heatsealing strength, shrinkage, thickness irregularity, aromaretainability, and the adsorptivity were also excellent, a requirementof the present invention was not satisfied for having low tensilebreaking strength. The film of Comparative Example 3 marked heat sealingstrength of zero, for having been peeled off immediately after heatsealed, due to a low ΔCp of the heat sealing layer.

INDUSTRIAL APPLICABILITY

The present invention relates to a polyester film excellent in heatsealing strength, which is capable of being suitably used in sealantuse, not only for being excellent in heat sealing strength, but also forbeing less liable to adsorb various organic compounds, and for having asmall shrinkage in heating, and high strength. At least one layerincluding the polyester film of the present invention is also capable ofbeing laminated with another film, and it is also possible to providepackaging bag including such a laminate.

The invention claimed is:
 1. A polyethylene terephthalate film whichsatisfies the following requirements (1) to (6): (1) the film has atleast one heat sealing layer A, and wherein the heat sealing layer A ispresent on at least one of the surfaces of the film; (2) the film has apeeling strength of 4 N/15 mm or more and 25 N/15 mm or less when twosamples of the heat sealing layer A are heat-sealed together at 160° C.and 0.2 MPa for 2 seconds; (3) the heat sealing layer A of thepolyethylene terephthalate film has a difference in reversing heatcapacity of 0.18 J/g·K or more and 0.35 J/g·K or less, and wherein thedifference in reversing heat capacity is measured between at a lower anda higher temperature than a glass transition temperature with atemperature modulated DSC; (4) the film has a heat shrinkage of −1.5% ormore and 15% or less both in a longitudinal direction and in a widthdirection when treated in hot water at 80° C. for 10 seconds; (5) thefilm has a tensile breaking strength of 80 MPa or more and 300 MPa orless either in a lengthwise direction or in a transverse direction, and(6) the film is a single layer of the heat sealing layer A or a laminatestructure comprising the heat sealing layer A and a layer B other thanthe heat sealing layer A, wherein a polyester component constituting thepolyethylene terephthalate film comprises one or more amorphous unitsderived from (i) one or more monomer components selected from the groupconsisting of terephthalic acid and isophthalic acid and (ii) one ormore monomer units selected from the group consisting of neopentylglycol, 1,4-cyclohexanedimethanol, and diethylene glycol, the heatsealing layer A constitutes a component comprising ethyleneterephthalate and said one or more amorphous units, and the amount ofsaid one or more amorphous units in the heat sealing layer A is 17.0 mol% or more and 30.4 mol % or less, and the amount of ethyleneterephthalate unit in the heat sealing layer A is 56.0 mol % or more,and the layer B constitutes a component comprising ethyleneterephthalate and said one or more amorphous units, and the amount ofsaid one or more amorphous units in layer B is 9.9 mol % or more and 25mol % or less, and the amount of ethylene terephthalate unit in thelayer B is 48.1 mol % or more.
 2. The polyethylene terephthalate filmaccording to claim 1, wherein a polyester component constituting thepolyethylene terephthalate film comprises units derived from1,4-butanediol.
 3. The polyethylene terephthalate film according toclaim 2, having a film thickness of 5 to 200 μm.
 4. A packaging bagincluding at least one of the polyethylene terephthalate film accordingto claim
 2. 5. A laminate comprising at least one layer of thepolyethylene terephthalate film according to claim
 2. 6. A packaging bagincluding at least one of the laminate according to claim
 5. 7. Thepolyethylene terephthalate film according to claim 1, having a filmthickness of 5 to 200 μm.
 8. A packaging bag including at least one ofthe polyethylene terephthalate film according to claim
 1. 9. A laminatecomprising at least one layer of the polyethylene terephthalate filmaccording to claim
 1. 10. A packaging bag including at least one of thelaminate according to claim
 9. 11. The polyethylene terephthalate filmaccording to claim 1, wherein the film further contains layer C, whereinlayer C constitutes a component comprising ethylene terephthalate andoptionally said one or more amorphous units, and the amount of said oneor more amorphous units in layer C is 1.0 mol % or less.
 12. Thepolyethylene terephthalate film according to claim 11, wherein the filmhas a thickness of 5 to 200 μm.
 13. A packaging bag including at leastone of the polyethylene terephthalate film according to claim
 11. 14. Alaminate comprising at least one layer of the polyethylene terephthalatefilm according to claim
 11. 15. A packaging bag including at least oneof the laminate according to claim 14.